Electrically augmented filtration devices and processes are in commercial operation, but those in use have involved immersing anodic and cathodic electrode assemblies in a slurry bath, depositing a cake on certain of the electrode assemblies and thereafter removing the cake, as product, from those assemblies by means such as doctoring or blow-back. Quite often, however, the desired final product is a slurry, rather than relatively dry cake, so that the cake product must be reslurried to the desired moisture content; a retrogressive process step, so to speak. Then too, the commercial process just described has some of the characteristics of a batch-type operation; each of the electrode assemblies on which cake is to be deposited, as stated above, passing through a cycle of immersion, cake deposition, cake doctoring and reimmersion. Such batch-type processes tend to require rather complex and expensive mechanical equipment and frequent maintenance attention.
Process and apparatus for carrying out dewatering of suspensions are disclosed in U.S. Pat. No. 4,107,026, issued Aug. 15, 1978, U.S. Pat. No. 4,168,222, issued Sept. 18, 1979, U.S. Pat. No. 4,170,529, issued Oct. 9, 1979, U.S. Pat. No. 4,207,158, issued June 10, 1980, U.S. Pat. No. 4,303,492, issued Dec. 1, 1981, U.S. Pat. No. 4,419,209, issued Dec. 6, 1983, and copending application, U.S. Ser. No. 222,057, filed Jan. 2, 1981. The entire disclosures of these patents and application are incorporated herein by reference thereto.
In these earlier disclosures, self-contained hollow electrode assemblies are provided which are normally submerged in the suspension, but bodily removable therefrom for inspection or other purposes. These hollow electrode assemblies comprise two types of wall surfaces: ion-pervious walls for electrode assemblies of one polarity and liquid-pervious walls for electrode assemblies of the opposite polarity.
In operation, with electrode assemblies immersed in the suspension, a source of vacuum is connected to the interior of the electrode assemblies having liquid-pervious walls to provide a controllable pressure differential, thereby producing a flow of carrier liquid through the filtration surfaces, while the solids migrate in the opposite direction, under the influence of the electric field, to deposit as cake upon the electrode assemblies having ion-pervious walls. Filtrate or permeate liquid, that is, carrier liquid freed of solids, is withdrawn or pumped from the interior of the liquid-filled hollow electrode structure at a controllable rate.
As indicated, cake deposition occurs on the hollow electrode assemblies having ion-pervious surfaces: these electrode assemblies are filled with an electrolyte and have an electrode element therein immersed in the electrolyte. These electrode elements are thus isolated from direct contact with the suspension. The electrolyte is specially selected for high conductivity and compatibility with the electrode element. By compatibility is meant the relatively non-corrosive character of the electrolyte under the conditions that ordinarily prevail within the hollow electrode assembly. Since decomposition or evolution products and heat are generated at the electrode element within the hollow ion-pervious assembly, provision is made for a flow of electrolyte into and through the electrode chamber so that foreign products, including gases, and heat are swept out of the chamber and a relatively constant predetermined electrolyte composition is maintained.
The ion-pervious wall of the electrode assembly in these prior art structures comprises a chemically neutral filter media, ion exchange membrane, solid polymeric electrolyte, permeable porous membrane or structures incorporating combinations thereof, which, if film-like in nature or otherwise requiring support, may be backed by a chemically and electrically neutral grid so that a planar deposition surface is presented to the slurry being treated. Since the cake will form on this deposition surface member during electrofiltration and must be removed by contact with doctoring blades, a friction cage or spacing means may be provided to protect the somewhat fragile surface member from direct contact with the doctoring blades. The friction cage comprises a thin, open screenwork of relatively hard material covering the deposition surface member for contacting the doctor blades, while spacing means may comprise strips of plastic materials, such as Delrin acetal resin arranged in a frame-like configuration and of a thickness sufficient to prevent contact between the doctor blade and the deposition surface. For the purpose of cake recovery, the electrode assembly is raised to a position of emergence from the suspension, with the layer of collected solids or cake layer adhering thereto. Since the electrolyte remains within the electrode assembly in the raised position, a vacuum is applied internally of the electrode to reduce the pressure on the deposition surface member and so prevent rupture of the member. When the electrode assembly is immersed in operation, the vacuum applied internally serves to remove gaseous products, such as chlorine, oxygen or carbon dioxide evolved at the electrode element.
A somewhat different filtration process, crossflow filtration, has been proposed in the past and the following articles describe this concept:
J. D. Henry, "Cross-Flow Filtration," Recent Developments in Separation Science, Vol. 2, pages 205-224, 1972, discusses the application of a flow tangential to the filter medium to minimize accumulation of particles on the filter surface. The discussion does not extend to the use of an electric field to promote separation of solids in the slurry.
Henry et al, "A Solid/Liquid Separation Process Based on Cross-Flow and Electrofiltration," AICHE Journal, Vol. 23, No. 6, pages 851-859, November, 1977, discusses a mathematical model and experimental data for electrofiltration in a crossflow mode of suspensions of kaolin clay or oil emulsions from an aqueous solution. This paper does not describe protection means for an electrode in the process.